Shallowest Coalbed Methane Accumulation in the West: It’s All About Water and Microbes in the Powder River Basin
Gary D. Stricker1, Romeo M. Flores1, Donald A. Klein2, Cynthia A. Rice1,
and Raleigh A. Schmidt1
1 U.S. Geological Survey, Denver, CO
2 Colorado State University, Fort Collins, CO
Coalbed methane (CBM) accumulations in the Powder River Basin are at relatively shallow depths ranging from 180 to 2,500 feet. Studies indicate that regional uplift and erosion, beginning about 10 Ma, established aquifer systems in the Paleocene Fort Union Formation coal beds that facilitated production and retention of biogenic gas by methanogenic bacteria. Isotopic values of methane (δ13C from -51 to -83 per mil) and deuterium (δD) (-283 to -327 per mil) indicate methyl-type fermentation. Sulfate-depleted CBM water (Na-HCO3 type), with δD from -121 to -167 per mil, compared to δD values of methane indicates CO2 reduction. This mixed origin of the biogenic gas developed as generation was occurring during the late stages of burial.
Additional evidence of a biogenic origin of gas is shown from studies of deoxyribonucleic acid (DNA) in coal samples. DNA from the Wyodak-Anderson coal beds was cloned and sequenced using polymerase chain-reaction (PCR) controls. Seven methanogen primer-based sequences were separated and cultured, forming a separate, deeply rooted group of methanogens. The sequences did not give significant matches with known methanogenic sequences, implying they were molecular remnants of undescribed methanogens. This DNA analysis, coupled with study of phospholipid fatty-acids biomarkers derived from sulfate-reducing, methanogenic, and methanotrophic bacteria in submodern (older than 1952; tritium < 0.8 TU) co-produced water, supports methanogenic origin of the gas.
Gas preservation in the Fort Union coal reservoirs is controlled by hydrostatic pressures, which range from 0 to 300 psia in the shallow to deeper parts of the basin. Finally, the loss of CBM in shallow areas of the basin was caused by drop of hydrostatic pressure due to mining, whereas original pressure is retained in deeper areas.